Association between Catechol-O-Methyltransferase Val158Met (158G/A) Polymorphism and Suicide Susceptibility: A Meta-analysis

Background: Common functional Val158Met polymorphism in the Catechol-O-methyltransferase (COMT) gene may have an impact on an individual’s susceptibility to suicide, but individually published results are inconclusive. Therefore, we performed this meta-analysis to provide a more precise estimation of the association between COMT 158G/A (COMT Val158Met) polymorphism and suicide susceptibility. Study design: A cross-sectional study. Methods: This systematic review and meta-analysis is a comprehensive literature search of PubMed, Scopus, Web of Science and Google Scholar databases was conducted on case-control studies published up to Mar 2017. Crude odds ratios (ORs) with 95% confidence intervals (CIs) were calculated. Results: We identified 14 eligible case-control studies, including 2353 suicide attempters and 2593 controls. The pooled results indicated that COMT 158G/A (COMT Val158Met) polymorphism was not significantly associated with increased overall suicide risk. The same results were revealed based on ethnicity, Hardy–Weinberg equilibrium (HWE) status and genotyping technique. However, there was significant association between COMT Val158Met polymorphism and suicide risk among females under the homozygote (AA vs. GG: OR=1.829, 95% CI=1.158-2.889, P=0.010) and recessive (AA vs. AG +GG: OR = 1.787, 95% CI=1.195, 2.671, P=0.005) models, but not among males. Conclusions: COMT 158G/A (COMT Val158Met) polymorphism was associated with suicide susceptibility only in females.


Introduction
uicidal behavior is one of the main public health problems worldwide, which include a wide spectrum of self-destructive actions 1 . Approximately, one million people die by suicide each year worldwide. With a prevalence rate of 0.0145% and suicide accounting for 1.5% of death by all causes, it is the 10th leading cause of mortality worldwide 2 . The etiology of suicide due to a combined effect of genetic and external factors is not known exactly 3 . Social factors alone cannot fully explain suicidal behavior. However, some studies have indicated a strong genetic component to susceptibility to suicidal behavior 4 .
Catechol-O-methyltransferase (COMT) is an enzyme metabolizing noradrenaline in the synaptic cleft 5 . Association studies of dopamine receptor genes and COMT in schizophrenia and/or ADHD subjects report genetic associations with various domains of cognitive function, including executive function, verbal and working memory, attention, and performance monitoring. Therefore, it was suggested that COMT could also play a role in the susceptibility to suicidal behavior 6,7 . The COMT gene has a common functional polymorphism, val158met (rs4680) variant 8,9 . This polymorphism is due to a G to A transition at codon 158 of the membrane-bound form of COMT, which corresponds to codon 108 of the soluble form of COMT, resulting in a valine (Val) to methionine (Met) substitution 9 .
The genetic component of suicidal behavior was originally founded by family, adoption and twin studies. Its heritability is in the range between 30%-55% 10,11 . A family history of suicide is a strong risk factor for suicide and suicide attempt 11 . The COMT gene is located on the long arm of chromosome 22 at 22q11; spans 28 kb and contains six exons 9 . The function of COMT polymorphism in several psychotic conditions, such as schizophrenia, bipolar disorder, major depressive disorder, obsessive-compulsive disorder, and Parkinson's disease, and the putative effects of COMT on suicide have been proposed 12,13 .
Several case-control studies have reported the association between COMT polymorphism and suicide susceptibility 8,9,18,29 , but the results are inconclusive partially because of the possible small effect of the polymorphism on suicide risk and the small sample in some published study. Therefore, we performed a meta-analysis to derive a precise estimate of this association.

Publication Search
We searched the electronic PubMed, Scopus, Web of Knowledge, and Google scholar database to identify all casecontrol studies examined the association between COMT Val158Met (158G/A) polymorphism and suicide risk up to Mar 2017. The keywords were as follows: ''polymorphism'', ''genotype'', ''variant'', ''mutation'', suicide'', and ''suicide behavior''. Additionally, reference lists of the relevant casecontrol studies, review, and meta-analysis were reviewed manually to identify additional relevant studies.

Inclusion and Exclusion Criteria
Studies included in our meta-analysis had to meet all of the following criteria: 1) used case-control study or cohort study; 2) the studies evaluated the associations between the COMT Val158Met (158G/A) polymorphism and suicide risk; 3) studies published in English and Persian; and provided sufficient information for calculation of odds ratio (ORs) with 95% confidence interval (CI). Accordingly, the following exclusion criteria were also considered: 1) those studies that were not designed as case-control or cohort studies; 2) control population including suicide attempter or other similar conditions; 3) there was no available genotype or allele frequency; 4) duplicate of previous publication; and 5) studies not written in English or Persian. Moreover, if multiple studies from the same case series were available, the one including the more individuals was used in the analysis.

Data Extraction
Information was carefully extracted from all eligible publications independently by two investigators according to the inclusion criteria listed above and data with discrepancies in identification were discussed by all authors. For each included study, the following information was collected: first author's name, year of publication, country of origin, total number of cases and controls, distribution of genotypes and alleles, minor allele frequencies (MAFs), and P-value for Hardy-Weinberg equilibrium (HWE). The countries of origin were categorized as Asian, Caucasian, and African.

Statistical Analysis
The associations between the COMT Val158Met (158G/A) polymorphism and suicide were measured by odds ratios (OR) with 95% confidence intervals (CI) under the allele model (A vs. G), the homozygote model (AA vs. GG), the heterozygote model (AG vs. GG), the dominant model (AA+AG vs. GG), and the recessive model (AA vs. AG +GG). The heterogeneity assumption was checked by the Q-statistics with P-values < 0.1, and I 2 statistics was calculated to quantify the proportion of the total variation across studies due to heterogeneity 14 . I 2 values of 25%, 50%, and 75% were nominally assigned as low, moderate, and high estimates, respectively. When heterogeneity was considered significant, the random-effects model (DerSimonian-Laird approach) was performed. Otherwise, the fixed-effects model (Mantel-Haenszel approach) was used 15 . The departure from the Hardy-Weinberg equilibrium (HWE) for the control group in each study was tested using an online HWE calculator. The HWE was considered statistically significant when the P-value was less than 0.05. Sensitivity analysis was also tested by removing one study at a time to calculate the overall homogeneity and effect size. Publication bias was evaluated by funnel plots and further assessed by Egger's linear regression test 16,17 . All the statistical analyses were performed by comprehensive meta-analysis (CMA) ver. 2.0 (Chicago, IL, USA) software (Biostat, USA). All the P values were 2-sided. P-value less than 0.05 was considered statistically significant.

Characteristics of Studies
Thirty-seven publications fulfilled our search criteria were preliminarily were enrolled. After reading the titles and abstracts, 22 studies with duplicate titles, review, metaanalyses or focused on unrelated condition were excluded. One study was excluded because was not case-control. Finally, 14 case-control studies 8,9,18-29 on COMT genotypes and suicide risk were identified, including a total of 2353 suicide attempters and 2593 controls from 2000 to 2016 (Table 1). All studies were published in English. Among them five studies were in North America (USA, Canada, and Mexico), five in Asia (Taipei, Japan, and North Korea), and six (Germany, Croatia, Slovenia, Finnish Switzerland and France) in Europe. The genotypes in the healthy control group for two studies were not consistent with HWE (P<0.05) 23,29 . Table 2 summarizes the difference of genotypes and alleles frequency between males and females suicide attempters. Table 3

Subgroup Analysis
Stratified analysis by ethnicity showed no association between COMT Val158Met (158G/A) polymorphism and suicide risk in the Caucasian, Asian and mixed populations. The same results were also revealed by HWE status ( Table 3). Results of the stratified analysis studies for female and male attempters are summarized in Table 3. In the stratified analysis for male attempters, the data on genotypes of the COMT Val158Met (158G/A) polymorphism was available in six studies including 662 cases and 745 controls. However, the COMT Val158Met (158G/A) polymorphism was not significantly associated with suicide risk under all genetic models. For female attempters, the data on genotypes of the COMT Val158Met polymorphism was available in six studies including 554 cases and 540 controls. The COMT Val158Met (158G/A) polymorphism was associated with an increased the risk of suicide under the homozygote model (AA vs. GG: OR=1.829; 95% CI: 1.158, 2.889, P=0.010) and the recessive model (AA vs. AG +GG: OR=1.787; 95% CI: 1.195, 2.671, P=0.005) in females (Table 4). Additionally, there was no association between COMT Val158Met (158G/A) polymorphism and suicide risk by genotyping technique (Table 4).

Sensitivity Analysis
To evaluate the robustness of the association results, the meta-analysis was performed repeatedly with each study removed. No individual study significantly affected the pooled ORs. Sensitivity analysis was performed based on HWE, and the corresponding pooled ORs were not materially altered, indicating that our results were statistically robust.

Publication bias
Publication bias was qualitatively examined using the funnel plots and quantitatively estimated by Begg's test and Egger's test. For all genetic models, the shapes of the funnel plots did not reveal any evidence of obvious asymmetry (Figure 1 Table 1).

Discussion
To date, the association between COMT Val158Met polymorphism and suicide remains controversial because the reports include both positive and negative findings 9,30 . Several large studies in the USA, Germany, Slovenia, and Croatia suicide attempter cases found no evidence for an increased risk of suicide 21,24,26 . These epidemiological studies results are inconsistent because this association was found in presence of the COMT Met allele in the male and female suicide attempters and diagnosed with different mental and mood disorders such as schizophrenia and alcohol dependency.. These differences among studies might be explained by the different diagnostic entities used. Therefore, the main limitation could be that the genetic association studies on COMT 158G/A polymorphism have been performed in different ethnicity and may be different criteria have been applied in the studies to define suicidal behavior, which reduces power to find consistent results 21,24,31 .To the best knowledge, the present meta-analysis is the most comprehensive and accurate meta-analysis assessed the association between the COMT 158G/A (COMT Val158Met) polymorphism and suicide risk. Our results did not show any significant association between the COMT Val158Met polymorphism and suicide in overall. Several issues should be further discussed to address the negative results. However, for most 14 studies involved, when they were individually removed from the analysis, the association between COMT Val158Met polymorphism and suicidal behavior was not significant still. Thus, the evidence for an association between COMT and suicidal behavior was robust.
Our results were consistent with a more recent metaanalysis on COMT Val158Met and suicide risk. Eleven casecontrol studies and one case-only study were included on COMT Val158Met polymorphism 9 . COMT Val158Met polymorphism was not associated with suicidal behavior risk. However, COMT Val158Met and suicide risk essentially remain an open field, as meta-analysis study not selected healthy controls (with only 875 adult suicide attempters). Therefore, their results' reliability and the number of studies are considerably smaller than that needed to receive the robust conclusions 32 . Moreover, compared with their meta-analysis, subgroup analysis by ethnicity and gender were also carried out. In our meta-analysis, the COMT Val158Met polymorphism was associated with suicide risk in females.
Interestingly, when all the eligible studies were pooled into male and gender subgroups analysis, only in female subgroup increased risk for suicide was found. However, there was an imbalance in gender distribution, with more males in attempters than females. None of the genetic models showed association with male susceptibility, suggesting that COMT 158G/A (COMT Val158Met) manifest female specific influences on suicide attempt. Estrogen in females modulates neurotransmission and neuronal excitability of catecholaminergic systems and increases the concentration of dopamine in the synaptic gap, leading to the different influence of dopamine on emotion and motivation among male and females 33,34 . In addition, gender-related differences in suicide behavior might be due to the fact that COMT 158G/A (COMT Val158Met) polymorphism may differently influence personality traits that presumably affect suicide behavior 35,36 . However, we were unable to conduct accurate more analyses of COMT polymorphisms by gender subgroup for insufficient data, thus the existence of gender-specific influences on suicide in this meta-analysis remains unclear. In this metaanalysis the female specificity in effect of the COMT 158G/A (COMT Val158Met) polymorphism on suicide attempt could be a chance finding, this result is inconsistent with gender differences in the dopamine system 37 . Further examination in the included eligible case-control studies in the gender subgroups showed significant differences between cases and controls numbers in male and female subgroups.
In interpreting results of this meta-analysis, some limitations should be acknowledged. First, not all the eligible studies were collected; the sample size of the included studies was large enough by ethnicity, especially in non-Caucasian and Asian populations. Therefore, there was a lack of statistical power to evaluate better the association between COMT 158G/A (COMT Val158Met) polymorphism and suicide, especially in subgroup analysis. Second, due to limited individual data, a more precise analysis on other covariates such as age, number of suicide attempts, and environmental factors were not performed. Third, since the genotypes frequency of COMT 158G/A polymorphism not mentioned by gender of suicide attempters in all studies, we only evaluated the eligible studies for gender subgroups. Finally, as suicide is a complex trait, gene-gene and geneenvironment interactions were not analyzed. Specific environmental and lifestyle factors will alter the associations between gene polymorphisms and suicide susceptibility. Therefore, it is necessary to evaluate the roles of lifestyles in suicide.

Conclusions
The present meta-analysis is the most comprehensive and accurate meta-analysis assessed the association between the COMT 158G/A (COMT Val158Met) polymorphism and suicide risk. COMT Val158Met polymorphism was not associated with suicide risk in overall population. However, subgroup analysis by gender showed that COMT Val158Met polymorphism was strongly associated with suicide risk in the female subgroup. More comprehensive studies and larger samples are necessary to determine conclusively an association of COMT 158G/A (COMT Val158Met) with suicidal behavior.